Pressure Altimetry using the MPL3115A2 - Freescale

Figure 2. Zero-pressure reference (vacuum pressure) sealed inside the reference chamber
Figure 3 shows the cross section of the absolute-pressure sensing element of the MPL3115A2. The
difference in positive pressure between P1 and vacuum pressure causes the diaphragm to deflect inward.
The four piezoresistive strain-gauge resistors located in the diaphragm itself are connected as a
Wheatstone bridge, which detects the deflection of the diaphragm as a mechanical stress and provides a
voltage output. This voltage output is proportional to P1 and the supply voltage, which will be sampled by
the dedicated ASIC of the MPL3115A2 for further processing, such as signal amplification and ADC
conversion. Also the ASIC performs oversampling trim compensation, data-path calculations and I 2 C port
control to provide final digital outputs.
Figure 3. Cross section of absolute pressure-sensing element
Depending on the operation mode selected, the MPL3115A2 can directly output 20-bit
atmospheric-pressure data in Pascals or 20-bit altitude data in meters. Atmospheric pressure can be
resolved in fractions of a Pascal and altitude can be resolved in fractions of a meter. The internal
conversion of atmospheric pressure to altitude is based on Equation 3 derived from U.S. Standard
Atmosphere, 1976. Also, 12-bit temperature data in degree celsius is provided at the same time, as well as
for internal compensation of the pressure sensor.
Pressure Altimetry using the MPL3115A2, Rev. 0
6 Freescale Semiconductor, Inc.